1. Field of the Invention
The present invention relates to a liquid state detecting sensor for detecting the state of a liquid, and more particularly to a liquid state detecting sensor having a liquid concentration detecting element for detecting the concentration of a particular component in a liquid.
2. Description of the Related Art
In recent years, there are cases where an NOx selective reduction catalyst (SCR) system is used in an exhaust gas purifying apparatus for reducing and rendering harmless nitrogen oxides (NOx) emitted from a diesel engine, e.g., a motor automobile with a diesel engine mounted thereon. In this system, a urea aqueous solution is used as a reducing agent, but in order to effectively reduce the nitrogen oxides in this system, the concentration of urea in the urea aqueous solution is necessarily controlled within a prescribed range.
However, even in the case where a urea aqueous solution of the prescribed concentration is accommodated in a urea tank for storing the urea aqueous solution, there are cases where the urea concentration changes over time. In addition, there may be instances where a liquid other than the urea aqueous solution of the proper urea concentration, such as service water, is erroneously poured into the urea tank.
Accordingly, a system has been proposed in which a concentration sensor for monitoring the urea concentration of the urea aqueous solution is installed in the urea tank, and an alarm or the like is issued when an abnormality is detected. As such, the system informs a driver that the reduction (purification) of nitrogen oxides in the exhaust gas purifying apparatus is not being carried out properly (refer to JP-A-2000-371831).
3. Problems to be Solved by the Invention:
Incidentally, in detecting the concentration of a particular component such as urea in a liquid such as a urea aqueous solution using a liquid concentration detecting element, there are cases where bubbles or groups of bubbles (hereafter, these are also collectively referred to as the bubbles) can hinder the measurement. Namely, if the bubbles are in contact with the liquid concentration detecting element when measuring the concentration of a particular component of the liquid, defects result in which, for example, the flow of liquid around the liquid concentration detecting element is hindered and the presence of the bubbles influences the measurement of liquid properties.
On the other hand, there are cases where an enclosing member is provided around that portion of the liquid concentration detecting element which is disposed in the liquid, so as to control the liquid flow and protect the liquid concentration detecting element. In the case where such a construction is provided, if a multiplicity of bubbles accumulate in a region enclosed by the enclosing member, the periphery of that portion of the liquid concentration detecting element which is disposed in the liquid is surrounded by the multiplicity of bubbles. Consequently, it becomes difficult to detect the concentration, and the effect of the bubbles on the concentration measurement is particularly likely to occur.
In addition, there are also cases where an outer enclosing member is further provided around the enclosing member so as to control the liquid flow and protect the liquid concentration detecting element. In the case where the liquid state detecting sensor is thus constructed, it is necessary to give consideration not only to the bubbles which have entered the enclosed region, but also to the bubbles located between the enclosing member and the outer enclosing member.